I see what you mean. Unforuntately it seems to be something inside Kendo UI that’s causing mouse events to stop being passed to the diagram.
If you try: Kendo UI® Dojo by Progress
it will simply work (I removed Kendo from the sample).
I’m not sure this is something I can “fix” inside GoJS. Is there some way you could isolate the GoJS div from Kendo so that Kendo events don’t apply to it?